Prior art drying apparatus and methods have been utilized for drying organic products which are in the form of liquids or semi-liquids such as solutions and colloidal suspensions and the like. These prior art drying apparatus have been used primarily to produce various dried or concentrated foodstuffs and food-related products, as well as nutritional supplements and pharmaceuticals. The liquid products are usually first processed in a concentrator apparatus which employs a high-capacity heat source, such as steam or the like, to initially remove a portion of the moisture from the suspension. Then, the concentrated products are often processed in a prior art drying apparatus in order to remove a further portion of the remaining moisture.
Various types of prior art drying apparatus have been employed, including spray dryers and freeze dryers. While spray dryers are known to provide high processing capacity at a relatively low production cost, the resulting product quality is known to be relatively low. On the other hand, freeze dryers are known to produce products of high quality, but at a relatively high production cost.
In addition to spray dryers and freeze dryers, various forms of belt dryers have been used. Such prior art drying apparatus generally include an elongated, substantially flat, horizontal belt onto which a thin layer of product is spread. The product is usually either in the form of a concentrated liquid or a semi-liquid paste. As the belt slowly revolves, heat is applied to the product from a heat source. The heat is absorbed by the product to cause moisture to evaporate there from. The dried product is then removed from the belt and collected for further processing, or for packaging, or the like.
A typical prior art apparatus and method is disclosed in U.S. Pat. No. 4,631,837 to Magoon. Referring to FIGS. 1 and 2 of the '837 patent which are reproduced in the drawings which accompany the instant application as Prior Art FIGS. 1 and 2, an elongated frame or structure is provided on which an elongated water-tight trough 10 is supported. The trough 10 is preferably made of ceramic tile. An insulation layer 12 is provided on the outer surface of the trough 10. The interior surface of the trough 10 is lined with a thin polyethylene sheet 16. Parallel rollers 24, 26 are provided, with one roller being located at each end of the trough 10. One of the rollers 26 is driven by a motor.
A water heater 15 and circulation system, including a pump and related piping, is also provided with the prior art apparatus of the '837 patent. The water heater 15 is configured to heat a supply of water 14 to just below its boiling point, or slightly less than 100 degrees C. The pump and related piping system is configured to circulate the water 14 through the trough 10 so that a minimum given water depth is maintained throughout the trough. In addition, the water heater 15 and related circulation system is configured to maintain the water supply within the trough at a temperature which is slightly less than 100 degrees C.
A flexible sheet of polyester, infra-red transparent material 18 in the form of an endless belt is supported about the rollers 24, 26 at each end, and is also supported on top of the water supply 14 within the trough 10. That is, the polyester belt 18 is driven by the roller 26 and revolves there about and the roller 24, while floating on the water 14 within the trough 10. A thin layer of liquid product 20 is dispensed onto the revolving belt 18 by way of a product discharge means 28 which is located at an intake end of the apparatus.
As the layer of product 20 travels along the trough 10 on the belt 18 which floats on the water 14, the product is heated by the water 14 which is maintained near 100 degrees C., and on which the belt 18 floats. The heat from the water 14 drives moisture from the product 20 until the product reaches the desired dryness, whereupon the product is removed from the belt 18. The rate at which the belt 18 moves through the trough 10 can be regulated so that the product 20 will reach its desired dryness at the discharge end of the apparatus where it is removed there from.
Several characteristics of the drying apparatus and method disclosed by the '837 patent lead to inconvenient and troublesome use of the apparatus. For example, the trough 10 of a typical prior art apparatus as disclosed by the '837 patent has a length within the range of 12 to 24 meters or more. As a result, the apparatus occupies a relatively large amount of production space. Also, several potential problems regarding the operation of the prior art apparatus can be attributed to the use of water as a heat source.
For example, the prior art apparatus requires a relatively massive water heating and circulation system 15 for its operation. The water heating and circulation system 15 can prove troublesome in several ways. First, the water heating and circulation system 15 adds complexity to the configuration and construction of the apparatus as well as to its operation. The system 15 incorporates a water heater, a pump, and various pipes and valves which must all be maintained in a relatively leak-proof manner. The required water heating and circulation system 15 can also deter the ease of mobility of the prior art dryer because of the bulky nature of the system and because of the need for a water supply.
Secondly, the water 14, which is maintained below the boiling point can serve as a harbor for potentially dangerous microbial organisms which can cause contamination of the product 20. Thirdly, the presence of a large amount of water 14 can serve to counter the objective of the prior art apparatus which is to remove moisture from the product 20. That is, the water 14, by way of inevitable leaks and evaporation from the trough 10, can enter the product 20 thereby increasing the drying time of the product.
Moreover, because the water 14 is the sole source of heat for drying of the product 20, and because the water temperature is maintained below 100 degrees C., the process of drying of the product 20 is relatively slow. As a universally accepted rule, the quantity of heat transferred between two bodies is proportional to the difference in the temperature of each of the bodies. Also, as a general rule, the moisture contained in the product to be dried must absorb a relatively great amount of energy in order to vaporize. The product 20 initially contains a relatively high amount of moisture when it is initially spread onto the support surface 18. Thus, a relatively high amount of heat energy is required to vaporize the moisture and remove it from the product 18.
However, because the temperature of the water heat source of the prior art apparatus never exceeds 100 degrees C., the difference in the temperatures of the heat source and the product 20 is limited which, in turn limits the transfer of heat to the product. As the product 20 absorbs heat from the heat source, the temperature of the product will rise. This rise in temperature of the product as it travels through the apparatus results in an even lower difference in temperature between the product 20 and heat source which, in turn, further reduces the amount of heat transfer from the heat source to the product. For this reason, the prior art apparatus often requires extended processing times in order to satisfactorily remove moisture from the product 20.
Also, the prior art apparatus and method of the '837 patent does not provide for any flexibility in processing temperatures because the temperature of the heat source cannot be easily changed, if at all. For example, the production of some products can benefit from specific temperature profiles during the drying process. The “temperature profile” of a product refers to the temperature of the product as a function of the elapsed time of the drying process. However, because the temperature of the heat source of the prior art apparatus is not only limited to 100 degrees Centigrade, but also slow to change, the temperature profile of the product cannot be easily controlled, or changed.
Because the prior art apparatus disclosed by the '837 patent employs water as a heat source, and requires a large water heating system for its operation, the resulting prior art apparatus is large, heavy, immobile, complex, difficult to maintain, and can be a source of microbial contamination of the product. Additionally, because the temperature of the water heat source utilized by the prior art method and apparatus is limited to less than 100 degrees Centigrade, the prior art method of drying can be slow and inefficient, and does not provide for modification or close control of the product temperature profile.
Drying systems incorporating infrared heating elements can solve many of the problems of the prior art apparatus of the '837 patent. Such a drying system is disclosed in U.S. Pat. No. 6,539,645, which is incorporated herein by reference.
It is known that the wavelength band emitted from an infrared heater can be controlled by adjusting the temperature of the infrared heater. Increasing the temperature of an infrared heater will produce radiation of shorter wavelengths while decreasing the temperature of an infrared heater will produce radiation of longer wavelengths. Prior techniques for heating certain substances with infrared radiation have included selection of a particular wavelength band of infrared radiation that is most efficiently absorbed by the substance being heated and/or that produces a desired heating effect.
U.S. Pat No. 5,382,441, for example, discloses an infrared heating system for heating baked goods. The '441 patent discloses that known IR food processes control the source temperature of the heaters to adjust the wavelength of the radiation during a baking process. If greater surface heating is required, the source temperature is decreased to produce longer wavelengths that are less capable of penetrating the surface of the product. Conversely, if less surface heating is required, the source temperature is increased to produce wavelengths that are more capable of penetrating the surface of the product.
U.S. Pat. No. 5,974,688 discloses an infrared heating system for drying wastewater sludge. The system disclosed in the '688 patent purportedly maintains the source temperature of infrared heaters at a temperature that produces wavelengths in a range that maximizes the heat transfer rate into wastewater sludge, thereby minimizing drying time.
However, the prior art techniques of the '411 and '688 patents are insufficient for heating and drying applications where it is desirable to precisely control the temperature of the product being dried, for example, to heat the product according to a predetermined temperature profile that produces the best results for a particular product, such as when drying liquid food products. The need to maintain or control the temperature of the product being dried is directly at odds with the need to heat the product with radiation of a particular wavelength, such as to maximize the heat transfer rate. For example, if the product becomes too hot, then the temperature of the heater must be decreased to avoid overheating and/or burning the product, however decreasing the temperature will increase the wavelength of the radiation. Conversely, if the product requires more heat in a short amount of time to avoid underheating the product, then the temperature of the heater must be increased, which will decrease the wavelength of the radiation. As can be appreciated, the prior art techniques of the '411 and the '688 patents sacrifice the ability to control the temperature profile of the product by maintaining the heat sources at predetermined settings that produce radiant heat at the desired wavelength.